1. Field of the Invention
This invention relates to an optical glass having specific optical constants expressed by a refractive index (nd) of 1.66 to 1.77 and an Abbe number (.nu.d) of 30 to 50.
2. Description of the Prior Art
A number of glasses having optical constants of a refractive index and an Abbe number within the ranges described above have been known for a long time. Many of them, however, contain a relatively large amount of PbO, or both PbO and TiO.sub.2. PbO has the defect of poor chemical resistance. The copresence of PbO and TiO.sub.2 has the defect of causing a strong coloration. Moreover, such glasses tend to undergo greater phase separation as the TiO.sub.2 content increases. In order to avoid phase separation, relatively large amounts of B.sub.2 O.sub.3 and Al.sub.2 O.sub.3 must be used in the glass. Consequently, this causes a reduction in the amount of SiO.sub.2, and sufficient chemical resistance cannot be obtained.